Hybrid and module technology for high-reliability applications

Hybrid and module technology has been evolving for 50 years and now, in the form of Commercial-Off-The-Shelf (COTS) modules, is making major contributions to reducing design cycles and addressing size, weight and power challenges. Let’s take a look back over the history of the technology and explore some of the factors that are important to the military and aerospace industry.

Early hybridsIn the late 1950s, advances in computing using discrete transistors made huge strides forward, but circuit boards were becoming increasingly complex, sometimes with thousands of interconnected transistors, diodes, resistors and capacitors. A solution was needed to improve the density and reliability. Government agencies funded various hybrid circuit ideas.

The military was especially interested and active in this field. In 1958, the US Army funded RCA to develop their “micro-module” concept. This took the approach of uniform-sized cubes externally configured so that they could be clipped together. Internally, small wafers of the various discrete components were vertically stacked and interconnected at their edges. Component density by volume was more than doubled and reliability increased by a factor of six, which delighted the military who invested further over the coming few years. In 1962, according to an article in Electronic Engineering Times, a 10-component module cost $52, around 2.5 times the cost of a conventional discrete PCB solution.

Despite its cost, the RCA micro-module was very successful but short-lived and the birth of the integrated circuit (IC) no doubt contributed to its demise. At nine times the cost of a hybrid solution, early ICs were often the beneficiary of military or government funded programs and one notable 1962 project was the Raytheon-built Apollo Guidance Computer for NASA.

With the rapid evolution of ICs, their advantages over hybrids and modules were soon recognized. In that regard, it seems surprising that hybrid circuit technology survived at all. However, the military often had broader considerations, including product stability and long-term availability, reliability, utility versus innovation and complex operational requirements. These factors together with the specific technical advantages of hybrids and modules have undoubtedly contributed to their continued usage over the last 50 years.

IntegrationIn the timescale spanned by this article, ASIC technology has revolutionized the industry. Initially, gate arrays of a few hundred gates offered the military a route to improved digital integration and as gate densities rapidly advanced and development tools were refined, the days of the hybrid circuit looked numbered. In the late 1980s, military equipment designers recognizing the success of digital ASICs, attempted to apply the same techniques to mixed-signal circuits. Their motives were dominated by the need for miniaturization, as more and more complex systems were demanded by the military, which had big budgets at that time. But difficulties adapting full custom design tools for customer use and the complexity of analog design, meant that mixed-signal ASIC remained very resource intensive and highly reliant on semiconductor manufacturer design teams for, in effect, full-custom designs. While analog ASIC design tools and technology have made significant progress, the breadth of real-world analog problems are still difficult to implement with off-the-shelf semi-custom circuits. Thus, hybrid circuits have continued to offer a means of integrating various high-performance analog and signal-chain functions into a single package when off-the-shelf products will not do the job.